Heavy metal-oxide glass optical fibers for use in laser medical surgery and process of making

ABSTRACT

An improved optical glass fiber for transmitting mid infrared wavelength laser light in surgical instruments, includes a heavy-metal oxide component, preferably GeO 2  doped with heavier cations and anions, and which is capable of delivering at least three watts of laser power continuously for more than ten minutes, without failure. This glass fiber has an α(dB/m) at 2.94 μm of 10, preferably less, and can transmit at least 27% of the IR through a thickness of one foot.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a 371 of PCT/0593/06392, filed Jul. 02, 1993, which is a continuation-in-part of U.S. application Ser. No. 07/909,380, filed Jul. 6, 1992, now U.S. Pat. No. 5,279,778, the content of which is entirely incorporated herein by reference.

FIELD OF INVENTION

The present invention relates to improved IR transmitting fibers and, more particularly, to the use of oxide and heavy-metal containing glass fibers for coupling with a mid-infrared laser for medical laser surgical applications.

BACKGROUND OF THE INVENTION

Laser microsurgery and coronary angioplasty require precise removal of tissue without thermally damaging the surrounding tissues. Such medical/surgical lasers require using a wavelength in the mid-infrared wavelength region, i.e. between about 1.0 and 3.0 microns, which is the wavelength range most strongly absorbed by animal tissue. As the 2.94 micron wavelength of the Er:Yag laser is the most well-absorbed by animal tissue and the tissue absorption coefficient (α=1000cm⁻¹) is the highest with the Er:Yag laser, this laser is preferred for many surgical procedures (see "Laser Evolution", Nov. 1991). To efficiently use the power delivered by the Er:Yag laser, a ruggedized infrared (IR) optical fiber which transmits at 2.94 microns must be used. Coupled to the output of the laser beam, the IR fiber can deliver the power to ablate tissues.

Infrared transmitting zirconium fluoride based glasses such as ZrF₄ --BaF₂ --LaF₃ --AlF₃ --NaF--PbF₂ (see Esterowitz et al, "Angioplasty With a Laser and Fiber Optics at 2-94 μm") have emerged as the materials of choice for the IR transmitting fiber because their optical transmission between 2.5 microns to 5.0 microns exceeds 90 percent, as shown in FIG. 1. Similarly, fluoride glass fibers exhibit optical losses as low as 0.06 dB/m at 2.94 microns. This is equivalent to a transmission of over 90 per cent in a 30.5 cm long fiber. A flexible fiber hand-piece with a minimum length of about 15.24 cm and a desirable length of 30.5 cm or greater can be made adaptable to many laser delivery systems.

However, the zirconium fluoride based glass fibers have a drawback in that they are capable of carrying only very low power. Investigations recently carried out have shown the following: a 300 micron core fluoride glass fiber, when coupled to an Er:Yag laser operating with an output power of 180 mJ at a repetition rate of 10 Hz, could transmit around 135 mJ or 1.35 watts but failed after only two minutes. As the laser output power increased, the fiber damage occurred much faster. Fiber damage always occurred at the fiber input end face or along the fiber length resulting in a localized melt-down of the glass followed by power rupture. As a result, zirconium fluoride based glass fibers have a very limited use in conjunction with the Er:Yag laser, and in effect can only be used for a very short time because fiber damage quickly occurs.

It is known that AlF₃ used as a dopant for ZrF₄ base glass can increase the Tg of the glass considerably. However, the quantity of AlF₃ which can be incorporated as a dopant is very low, as AlF₃ tends to destabilize the glass and make it difficult to form into fibers. Even when formed, such fibers are not stable, as the resultant AlF₃ containing glass fibers tend to easily crystallize under localized heating which would inevitably occur with laser usage, thus inducing fiber damage and failure. Therefore, the addition of AlF₃ to the conventional ZrF₄ glass is not a solution to the problem of localized melt-down and resultant failure of the fiber. AlF₃ based glass of about 30 mol % AlF₃ is also known, but this glass is very unstable and difficult to form into fibers.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to overcome deficiencies in the prior art, such as indicated above.

Another object of the present invention is to provide improved IR transmitting optical fibers, especially useful for laser surgery.

A further object of this invention is to provide a family of optical fibers which are transparent to IR between 1 and 3 microns and which can be used with a surgical laser, e.g. an Er:Yag laser, which operates around the main absorption band of water and which can deliver at least 1.35 watts of laser power continuously for more than ten minutes, and which optical fibers can be used to transmit laser light for precise cutting of animal, e.g. human, tissues.

It has been determined according to the present invention that in order to prevent localized heating that would eventually damage the fiber, high temperature glasses having high glass transition temperature (Tg), thus high softening and high crystallization temperatures, and capable of transmitting at 2.94 microns, must be used as the fiber material. In addition, these glasses must have a high glass forming ability, i.e. a low tendency toward devitrification or crystallization. High grade and improved laser grade surgical fibers have therefore been developed according to the present invention based on three concepts: (1) proper choice of fiber material, i.e. high working temperature and stable glass, containing proper dopants to provide excellent IR transmission at 2.94 microns; (2) determining preferred processing techniques to enhance the 2.94 micron optical transmission; and (3) determining preferred techniques for producing the surgical fiber in large commercial quantities.

In general, the glasses of the present invention should be substantially water-free (less than 1.0 ppm and preferably less than 0.1 ppm of water), have a Tg of at least 290° C. and preferably at least 315° C., and a coupling efficiency¹ of at least 20% and preferably at least 35% and most preferably at least 50% over an optical path length of 30.5 cm; in this respect, the transmission should be at least 40% and preferably at least 50% over an optical path length of 30.5 cm. In addition, the glass should have a critical cooling rate, Rc, of less than 5.5° C./min and preferably less than 3.0° C./min, where Rc is defined as the slowest cooling rate at which a glass melt can be quenched without inducing crystallization. In this way the glass fiber will be able to deliver at least 1.35 watts of laser power continuously for at least five minutes and preferably more than ten minutes, efficiently and without failure.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a comparative graph showing transmission percent plotted against wavelength in microns for three different glasses;

FIG. 2 is a similar graph showing the results with pretreatment with fluorine;

FIG. 3-10 are similar graphs for other glasses and FIG. 11 is a schematic view of a device according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The preferred materials in the present invention are glasses based on or containing GeO₂, TeO₂, Sb₂ O₃, PbO, Bi₂ O₃, Al₂ O₃, P₂ O₅, Al(PO₃)₃, M(PO₃)₂ where M is Mg, Ca, Ba or Sr, and NPO₃ where N is Li, Na or K. Most preferred are those based on GeO₂. These oxide glasses, hereinafter referred to as heavy-metal oxide glasses, generally exhibit a glass transition temperature (Tg) and a glass stability much higher than the Tg and the glass stability in zirconium fluoride based glasses. As a result, localized heating which damages the fiber is prevented.

However, glasses formed entirely of one of these preferred oxides, especially those of lower molecular weight, do have a drawback in that their multiphonon absorption edge or infrared cut-off edge is only slightly above 2.94 microns. This will undermine transmission at 2.94 microns especially in a fiber as long as 30.5 cm. The absorption of infrared energy depends on atomic vibrations and follows Hooke's law which may be expressed as:

    ναf/μ with λ=1/ν

where ν=absorbing frequency (cm-1)

λ=absorbing wavelength (microns)

f=force constant or bond strengths

μ=average mass of all ions To push the infrared absorption edge toward longer wavelengths, the forces of attraction between ions should be low, i.e. the mass of the ions should be high. To overcome this problem, the heavy-metal oxide glasses are doped with even heavier cations or anions or both to stabilize the heavy-metal oxide ions and reduce their vibrations at the atomic level when stimulated by IR. While it has been previously known that adding heavy metal ions to glass in general will push the infrared cut-off edge toward longer wavelengths, this concept has never been previously used, insofar as is known, to increase the transmission of IR at 2.94 μm in heavy-metal oxide glasses, especially in connection with laser usage.

The most preferred glass of the present invention is an oxide glass where GeO₂ is the glass former. A previously known GeO₂ fiber originated from the work of H. Takahashi and I. Sugimoto, "Decreased Losses in Germanium-Oxide Glass Optical Fiber Prepared by VAD Method", Japanese Journal of Applied Physics, Vol. 22, No. 3 (Mar. 83) pp L139-L140. This fiber was doped with 20 mol % antimony (Sb) and was intended for telecommunication applications at 2.4 microns, but the loss of transmission of light through the fiber (i.e. the attenuation) at 2-4 μm was high, i.e. 10 dB/m, so that it transmits only about 44% of the IR through a length of 30.5 cm. It also has about 12% reflection losses at both ends, thus reducing its coupling efficiency to only 32%. Its use with an Er:Yag laser would thus result in low power transmission.

To increase the transmission at 2.94 microns, it is necessary to push the infrared cut-off edge of the GeO₂ glass toward longer wavelengths. To do so, it is absolutely necessary to substitute part of GeO₂ with dopants containing either larger and heavier cations than Ge which has an atomic weight of 72.6, or larger and heavier anions than O (such as F, Cl, Br, I) or a combination of both. As regards the larger and heavier cations, it will be understood that a smaller quantity of a larger and heavier cation will accomplish the same result as a larger quantity of a cation which is not so heavy. Thus, smaller quantities of Pb, Bi, Te, Hf, La, Ba, etc., having an atomic weight of about 125 and greater will accomplish the same extension of the infrared cut-off edge as will larger quantities of cations having an atomic weight above 73 but less than about 125, such Zr, Sb, As, Sr, Ca, Cd, Y, etc. Thus, for example as noted above, the use of 20 mol % of Sb₂ O₃ as a dopant for GeO₂ (80 mol %), as in the case of the Takahashi et al glass, did not push the infrared cut-off edge as far as most desirable.

Dopants having either or both lighter cations and anions, such as ZnO or TiO₂, etc. can be added to the glass in small amounts to modify glass properties, such as stability, hardness, etc.; but the addition of lighter compounds does not contribute to the transmission of 2.94 micron IR. Typical germanate glass compositions of the present invention are shown in Table 1. As seen from Table 1, the examples set forth therein never contain more than 27 mol % of oxides or halides of light-weight cations, i.e. these examples contain GeO₂ together with a heavy metal dopant in a combined amount of at least 73 mol %; specifically, Example V contains 4 mol % ZnO, Example VIII contains 8 mol % NaF, Example IX contains 27 mol % CaF₂, and Example XI contains 20 mol % of AlF₃.

                                      TABLE 1                                      __________________________________________________________________________     TYPICAL GERMANATE GLASSES OF THE INVENTION                                     Types Of                                                                       Germanate                                                                      Glass                                                                               GeO.sub.2                                                                         PbO                                                                               PbF.sub.2                                                                         NaF                                                                               AlF.sub.2                                                                         TeO2                                                                              ZrO.sub.2                                                                         La.sub.2 O.sub.3                                                                   BaO                                                                               Bi.sub.2 O.sub.3                                                                   Sb.sub.2 O.sub.3                                                                   As.sub.2 O.sub.3                                                                   SrF.sub.2                                                                         ZnO.sub.2                                                                         CaF.sub.2                                                                         PbCl.sub.2               __________________________________________________________________________     I    43 57                                                                     II   61 24 15                                                                  III  60                                      40                                IV   40 56                4                                                    V    40 56                                      4                              VI   54  8                    38                                               VII  38 56                6                                                    VIII 43 49    8                                                                IX   73                                            27                          X    42  5                           53                                        XI   32 48       20                                                            XII  25 35                               40                                    XIII 80                          20                                            XIV  32 28          40                                                         XV   20 30                                            50                       XVI  50 30                           20                                        XVII 50    25                        25                                        XVIII                                                                               60                          20  20                                        XIX  18 79             3                                                       __________________________________________________________________________

From Table 1 it will be apparent that the amount of GeO₂ can vary considerably, dependent on the co-component dopants. If one or more of very heavy compounds such as PbO, PbF₂, PbCl₂, Bi₂ O₃ are used as the dopants, then the quantity of GeO₂ may even exceed 80 mol %, whereas if the dopants are of a lighter weight the quantity of GeO₂ should preferably not exceed about 75%. The excellent stability provided by GeO₂ diminishes when the content of GeO₂ becomes less than about 10 mol %. The suitability of various glass compositions can be determined by routine experimentation, based on the teachings of the present disclosure.

Besides the germanate glasses, there are other heavy-metal oxide glasses that are effective with the Er:Yag laser. The other heavy-metal oxide glasses of this invention include tellurate glasses, phosphate and fluorophosphate glasses, antimonate glasses, bismuth and lead glasses and aluminate glasses. These, however, suffer from one or more difficulties as pointed out below.

The tellurates are glasses which contain TeO₂ as the glass network former. TeO₂ glass by itself does not transmit well at 2.94 microns but the addition of dopants having larger and heavier cations than Te or larger and heavier anions than O, or both, results in a much higher 2.9 micron transmission. Examples of such a glass are 60TeO₂ -10PbO-30ZnF₂ and 20TeO₂ -20PbO-20ZnF₂ -40V₂ O₃. As noted above, dopants having lighter cations, such as V₂ O₃, can be incorporated into the glass to increase its stability against devitrification. The tellurate glasses have a drawback in that Te is a highly toxic chemical.

Glasses that contain P₂ O₅ as the glass former are referred to as phosphate glasses. Glasses that contain Al(PO₃)₃ or M(PO₃)₂ where M=Mg, Ca, Ba, Sr, or NPO₃ where N=Li, Na, K, are referred to as fluorophosphate glasses. Both of these glasses exhibit a large absorption band at 4.8 microns as a result of the P-O absorption (see FIG. 5). This absorption band will adversely affect the glass transmission at 2.94 microns especially in a 30.5 cm long fiber. Therefore, to optimize the fiber transmission at 2.94 microns, the (P-O) concentration in these phosphate and fluorophosphate glasses must be kept as small as possible. Examples of phosphate and fluorophosphate with small (P-O) contents are 10P₂ O₅ -60CaF₂ -30AlF₃ and 1.7Al (PO₃)₃ -38.3AlF₃ -10NaF-8MgF₂ -27CaF₂ -7SrF₂ -8BaF₂. In general, the (P-O) content should not exceed about 10 molt.

The antimonates are glasses that contain Sb₂ O₃ as the glass former. The infrared cut-off edge is at longer wavelengths for antimonate glasses than the comparable germanate glass since Sb is much heavier than Ge. Sb₂ O₃ glass by itself, however, exhibits a poor glass forming ability. To stabilize the Sb₂ O₃ glass, other oxide dopants such as Al₂ O₃ and R₂ O where R is Li, Na or K must be added. Examples of such glasses are 45Sb₂ O₃ -20Al₂ O₃ -35Na₂ O and 50Sb₂ O₃ -18Al₂ O₃ -26K₂ O-6Na₂ O. Note also that GeO₂ can also be incorporated into the Sb₂ O₃ glass to enhance its stability.

Antimonate glass is also receptive to dopants having anions which are heavier than O, i.e. F, Br, Cl and I. Such dopants will move the infrared edge of the glass toward longer wavelengths and therefore will enhance the glass transmission at 2.94 microns. Examples of such glasses are 80Sb₂ O₃ -20PbCl₂ ; 70Sb₂ O₃ -30PbBr₂ ; 70Sb₂ O₃ -30PbI₂ ; 80Sb₂ O₃ -20MnF₂ ; 80Sb₂ O₃ -20ZnF₂ ; 80Sb₂ O₃ -20ZnF₂ and 80Sb₂ O₃ -20SrCl₂.

Bismuth and lead glasses contain Bi₂ O₃ and PbO as glass network formers. Bi and Pb are large and heavy elements thus contributing to the high transparency of these glasses at 2.94 microns. However, bismuth and lead glasses have the drawback that they are relatively unstable and have high refractive indices which contribute to lower coupling efficiency than other heavy-metal oxide glasses. Dopants 30 having lighter cations such as Ga₂ O₃, CdO, SiO₂ are added to prevent crystallization. Examples of such glasses are 61Bi₂ O₃ -13Ga₂ O₃ -26CdO; 25Bi₂ O₃ -57.5PbO-17.5Ga₂ O₃ and 85PbO-15Al₂ O₃.

Aluminate glasses contain Al₂ O₃ as the primary glass former. Secondary glass formers include CaO, TiO₂, Nb₂ O₅ and Ta₂ O₅. Al is lighter than Bi, Pb, Sb, Te and Ge thus contributing to a lower transmission at 2.94 microns. The incorporation of dopants having larger and heavier cation than Al such as BaO, ZrO₂, PbO and Bi₂ O₃ or larger and heavier anion than O such as PbF₂ and AlF₃, or both, enhances the transmission at 2.94 microns; and because Al is so relatively light, in general the aluminate glasses should not usually contain more than about 60 mol % Al₂ O₃. Examples of such glasses are: 35.9Al₂ O₃ -59.4CaO-4.7BaO; 32.5Al₂ O₃ -55.2CaO-7.0BaO-5.3PbO; 27Al₂ O₃ -64CaO-7Bi₂ O₃ -2AlF₃ ; 20Al₂ O₃ -40Ta₂ O₅ -40K₂ O; 15Al₂ O₃ -42.5Nb₂ O₅ -42.5K₂ O; and 25.02Al₂ O₃ -19.83TiO₂ -19.83Ta₂ O₅ -4.12ZrO₂ -12.46BaO-9.37K₂ O-9.37Na₂ O.

The heavy-metal oxide glasses have another conventional drawback in that they invariably contain small amounts of water which are originally bonded to the raw oxide starting materials and/or which become incorporated into the glass during melting in the atmosphere. Since water absorbs strongest at 2.94 microns, trace amounts of it can undermine the ability of the glass to transmit at 2.94 microns, especially in a 30.5 cm or longer fiber, as this problem becomes increasingly severe as the path length through the glass increases.

To eliminate the presence of water and water absorption, the heavy-metal oxide glasses must be processed in a water-free atmosphere, e.g. a dry glove box atmosphere of argon or nitrogen. The weighing and batching of the chemicals, and the melting of the glass must be conducted in a dry atmosphere of less than 0.1 ppm water. Prior to the present invention, heavy-metal oxide glasses were generally processed in air, thus picking up moisture from the atmosphere.

It is known to use fluoride dopants such as PbF₂ or AlF₃ in heavy metal oxide glass to minimize the water content. Thus, in addition to dry melting according to the present invention, one or more halide compounds such as a fluoride and/or a chloride may be added to enhance the water removal. However, the use of a halide alone is not adequate. Another technique to enhance water removal and for the drying of the raw materials according to the present invention is to use a reactive gas such as fluorine or chlorine, at low temperatures of less than 100° C. to prevent conversion of oxide to fluoride or chloride.

Two techniques can be used successfully to fiberize the heavy-metal oxide glasses. The rotational casting process described by Tran et al in U.S. Pat. No. 5,055,120 (1991) is very efficient in making glass preforms from heavy-metal oxide glasses the viscosities of which are less than 50 poises at the melt temperature.

The second approach, called the double crucible method, as described in "Material Systems, Fabrication and Characteristics of Glass Fiber Optical Waveguide" by Merle D. Rigterink, Ceramic Bulletin, Vol. 55, No. 9 (1976), is preferred when the cladding glass melt cannot be rotated to form a perfect tube, i.e. when the melt viscosity is relatively high at the melt temperature.

The following examples, offered illustratively only, further explain the present invention. In the following examples all transmission curves were obtained from glass samples which were ground and polished to a thickness of 2 mm.

EXAMPLE 1

Used as starting chemicals were GeO₂, PbO and PbF₂. Twenty-five grams in total of these materials were used to prepare a germanate glass the final composition of which was 56GeO₂ -29PbO-15PbF₂. The chemical powder was weighed and mixed in a platinum crucible inside a glove box filled with argon. The water content of the glove box was less than 0.1 ppm. The crucible was capped with a platinum lid then placed inside an electric furnace located within the glove box and was heated to 1100° C. and soaked for three hours. The resulting molten glass was cast into a cylindrical mold, 1.5 cm in diameter, which was maintained at an annealing temperature of 330° C. The mold was then cooled slowly to room temperature. The glass disc thus obtained was used for characterization.

By way of comparison, (1) a 25g germanate glass containing 80GeO₂ -20Sb₂ O₃ according to Takahashi et al was prepared using the same procedure described above; and (2) a 25 g zirconium fluoride based glass containing 53ZrF₄ -18BaF₂ -3LaF₃ -3AlF₃ -18NaF-5PbF₂ was melted under the same conditions as above except that the temperature at which the components were melted was 875° C. and the temperature of the mold into which the mold was cast was 263° C.

Using a Perkin-Elmer DSCII differential scanning calorimeter, the glass transition temperatures (Tg) and the critical cooling rates of the three glasses were determined. As expected, the Tg's of the germanate glasses, 600° C. for the GeO₂ -Sb₂ O₃ glass and 330° C. for the GeO₂ -PbO-PbF₂ glass, were higher than that of the fluoride glass which was only 263° C. The Rc's of the two germanate glasses were about 2.5° C./min whereas the Rc of the fluoride glass was about 5° C./min.

The optical transmission curves plotted in FIG. 1 show the infrared edge of each of the three glass samples. The infrared cut-off edge is defined as the wavelength at which the transmission starts to decrease. The infrared cut-off edge of the Takahashi et al GeO₂ -Sb₂ O₃ sample is 4.1 microns (Curve B), and the closest to 2.94 microns. When heavy compounds of PbO and PbF₂ were substituted, the edge shifted toward 4.7 microns (Curve A) thus enhancing significantly the transmission at 2.94 microns in a long fiber. The fluoride glass (Curve C) is most transparent with an infrared cut-off edge of 5.8 microns, but such fluoride glass has too low a Tg.

It should be noted that dry melting alone did not eliminate water in the glass. This is shown by the large water absorption band located at 2.9 microns in the case of GeO₂ -Sb₂ O₃ glass sample (Curve B). The addition of fluoride such PbF₂, as in the case of the GeO₂ -PbO-PbF₂ glass sample, combined with dry melting, resulted in elimination of the water. In the later case, F reacted with OH from the water to form HF.

EXAMPLE 2

Used as starting materials were GeO₂ and Sb₂ O₃. Twenty-five grams in total of these oxides were used to prepare a heavy-metal oxide glass containing the Takahashi et al composition of 80GeO₂ -20Sb₂ O₃. Inside a glove box with an argon atmosphere of less than 0.1 ppm water, the chemical powder was weighed, mixed and then placed in a Teflon beaker. A reactive gas, fluorine in this case although other water-reactive gases can be used, was passed slowly through the powder via a small Teflon tube, about 5 mm inside diameter. The beaker was heated to around 70° C. After eight hours, the powder was transferred to a capped platinum crucible and melted using the same procedure of Example 1.

The transmission curve for the glass sample is plotted in FIG. 2. When compared with the transmission curve for the same compositional glass but without fluorine treatment (Curve B shown in FIG. 1), it is clear that fluorine treatment combined with dry melting enhances the water removal from the glass.

EXAMPLE 3

A 25 g germanate glass containing 30GeO₂ -50PbO-18AlF₃ -2PbF₂ was melted under the same conditions as in Example 1. Its transmission characteristics shown in FIG. 3 and its Rc value are very similar to those of the 56GeO₂ -29PbO-15PbF₂ glass of Example 1, except that its Tg of 380° C. is higher.

EXAMPLE 4

Used as starting materials were Sb₂ O₃, PbO, PbF₂ and GeO₂. Twenty-five grams in total of these chemicals were used to prepare a germanate glass containing 25Sb₂ O₃ -25PbF₂ -50GeO₂. The batching and melting procedures were similar to the ones used in Example 1. The transmission curve obtained for the glass is plotted in FIG. 4. The Tg and Rc of the glass obtained by differential scanning calorimetry was 310° C. and 2.5° C./min, respectively.

EXAMPLE 5

A 25 g tellurate glass containing 57TeO₂ -13PbO-28ZnF₂ -2PbF₂ was melted under the same conditions as in Example 1. The transmission curve obtained from the glass sample plotted in FIG. 5 shows an extended infrared cut-off edge as compared to that of the glasses of Examples 1 and 3, which was attributed to the heavier Te cation. Using differential scanning calorimetry, a Tg value of 350° C. and an Rc value of 3° C./min were obtained.

EXAMPLE 6

Used as starting materials were P₂ O₅, CaF₂, and AlF₃. Twenty-five grams in total of these materials were used to prepare a phosphate glass containing 60CaF₂ -26.5AlF₃ -3.5PbF₂ -10P₂ O₅. The batching and melting procedures were similar to the ones used in Example 1. The transmission curve for the glass plotted in FIG. 6 shows an absorption band at 4.8 microns due to P-O and an infrared cut-off edge at about 4 microns, making this glass barely suitable, i.e. suitable only for short length fibers of 15.25 cm or shorter. The Tg and Rc of the glass obtained by differential scanning calorimetry were 448° C. and 3° C./min respectively.

EXAMPLE 7

Used as starting materials were Al(PO₃)₃, AlF₃, ZrF₄, BaF₂, CaF₂, SrF₂, YF₃, MgF₂, NaF and PbF₂. Twenty-five grams in total of these chemicals were used to prepare a fluorophosphate glass containing 0.2Al(PO₃)₃ -29AlF₃ -10.2ZrF₄ -9.8BaF₂ -18.3CaF₂ -12SrF₂ -8.3YF₃ -3.5MgF₂ -3.8NaF-5PbF₂. The batching and melting procedures were similar to the ones used in Example 1. The transmission curve for the glass plotted in FIG. 7 shows an absorption band at 4.8 microns due to P-O. This absorption band is smaller than the one obtained for the phosphate glass of Example 5 because of a smaller (P-0) concentration. The Tg and Rc of the glass obtained by differential scanning calorimetry were 390° C. and 4.5° C./min, respectively. The presence of so much AlF₃, needed in this glass to increase the Tg to an acceptable level, tends to severely de-stabilize the glass, and the phosphate is needed to permit stable fiber manufacture.

EXAMPLE 8

Used as starting oxides were Bi₂ O₃, PbO, CdO and Ga₂ O₃. Twenty-five grams in total of the oxides were used to prepare a bismuth glass containing 61Bi₂ O₃ -26CdO-13Ga₂ O₃, and 25 g in total of the oxides were used to prepare a lead-bismuth glass acontaining 25Bi₂ O₃ -57.5PbO-17.5Ga₂ O₃. The batching and melting procedures are similar to the ones described in Example 2. The transmission curve for the f6rmer is plotted as curve B and for the latter is plotted as curve A in FIG. 8. The Tg's of the bismuth and lead-bismuth glasses, obtained by differential scanning calorimetry, were 375° C. and 350° C., respectively. The Rc's of both glasses were about 3.5° C./min.

EXAMPLE 9

Used as starting oxides were Al₂ O₃, Ta₂ O₅ and K₂ O. Twenty-five grams in total of these materials were used to prepare an aluminate glass containing 20Al₂ O₃ -40Ta₂ O₅ -40K₂ O. The batching and melting procedures were similar to the ones used in Example 2, except that the melt temperature was raised to 1500° C. The transmission curve obtained for the glass sample is plotted in FIG. 9. The Tg and Rc of the glass, obtained by differential scanning calorimetry, were 525° C. and about 5° C./min, respectively.

EXAMPLE 10

Used as starting oxides were Sb₂ O₃, Al₂ O₃, KNO₃ and Na₂ CO₃. Twenty-five grams in total of these materials were used to prepare an antimonate glass containing 50Sb₂ O₃ -18Al₂ O₃ -26K₂ O-3Na₂ O-3Li₂ O. The batching and melting procedures were similar to the ones used in Example 2. The transmission curve for the glass is plotted in FIG. 10. The Tg and Rc value obtained by differential scanning calorimetry was 403° C. and 4° C./min, respectively.

EXAMPLE 11

The germanate glass of Example 1, 56GeO₂ -29PbO-15PbF₂ (Fiber No. 1); the germanate glass of Example 2 (Fiber No. 2) 80GeO₂ -20Sb₂ O₃ ; the germanate glass of Example 3 (Fiber No. 3), 30GeO₂ -50PbO-18Alf₃ -2PbF₂ ; the germanate glass of Example 4 (Fiber No. 4), 25Sb₂ O₃ -25PbF₂ -50GeO₂ ; the tellurate glass of Example 5 (Fiber No. 5), 57TeO₂ -13PbO-28ZnF₂ -2PbF₂ ; the phosphate glass of Example 6 (Fiber No. 6), 60CaF₂ -26.5AlF₃ -3.5PbF2-10P₂ O₅ ; the fluorophosphate glass of Example 7 (Fiber No. 7), 0.2Al(PO₃)₃ -29AlF₃ -10.2ZrF₄ -9.8BaF₂ -18.3CaF₂ -12SrF₂ -8.3YF₃ -3.5MgF₂ -3.8NaF-5PbF₂ ; the bismuth glass of Example 8 (Fiber No. 9), 61Bl₂ O₃ -26CdO-13Ga₂ O₃ ; the lead-bismuth glass of Example 8 (Fiber No. 9), 25Bi₂ O₃ -51.5PbO-17.5Ga₂ O₃ ; the aluminate glass of Example 9 (Fiber No. 10), 20Al₂ O₃ -40Ta₂ O₅ -40K₂ O; and the antimonate glass of Example 10 (Fiber No. 11) 50Sb₂ O₃ -18Al₂ O₃ -26K₂ O-3Na₂ O-3Li₂ O were used as the core material for the making of optical fiber for laser surgery. By way of comparison, the fluoride glass of Example 1 (Fiber No. 12), 53ZrF₄ -18BaF₂ -3LaF₃ -3AlF₃ -18NaF-5PbF₂, was also used as a core material for the making of a fluoride glass optical fiber.

The cladding material for each fiber was the same type of glass as the core glass except the concentration of one or two components in each respective glass was altered to lower the glass refractive index. The core and cladding glass compositions of each type of fiber and its respective numerical aperture (NA) are given in Table 2. Twenty-five grams of each of the cladding glass were made using the same batching and melting procedures used for the core glass.

FIG. 11 schematically illustrates a device according to the present invention including a laser 10, such as one operating in the wavelength region of up to 4.5 microns for power transmission, and an optical fiber 12 for transmitting laser light generated by the laser 10, the optical fiber having a solid core 14 formed of a glass comprising a heavy-metal oxide according to the present invention, and a cladding 16.

                                      TABLE 2                                      __________________________________________________________________________     Core and cladding glass compositions used in the fabrication of                heavy-metal oxide glass fibers.                                                Fiber                                                                          No.                                                                               Glass Type                                                                             (Core)/(Clad) Composition (mol %)      NA                           __________________________________________________________________________     1  Germanate                                                                              (56 GeO.sub.2 -29 PbO-15 PbF.sub.2)/(61 GeO.sub.2 -24 PbO-15                   PbF.sub.2)                             0.20                         2  Germanate                                                                              (80 GeO.sub.2 -20 SB.sub.2 O.sub.3)/(100 GeO.sub.2)                                                                   0.48                         3  Germanate                                                                              (32 GeO.sub.2 -50 PbO-18 AlF.sub.3 -2 PbF.sub.2)/(32 GeO.sub.2                 -48 PbO-20 AlF.sub.3)                  0.19                         4  Germanate                                                                              (25 Sb.sub.2 O.sub.3 -25 PbF.sub.2 -50 GeO.sub.2)/(25 Sb.sub.2                 O.sub.3 -21.5 PbF.sub.2 -53.5 GeO.sub.2)                                                                              0.20                         5  Tellurate                                                                              (57 TeO.sub.2 -13 PbO-28 ZnF.sub.4 -2 PbF.sub.2)/(60 TeO.sub.2                 -10 PbO-30 ZnF.sub.2)                  0.20                         6  Phosphate                                                                              (60 CaF.sub.2 -26.5 AlF.sub.3 -3.5 PbF.sub.2 -10 P.sub.2                       O.sub.5)/(60 CaF.sub.2 -30 AlF.sub.3 -10 P.sub.2 O.sub.5)                                                             0.19                         7  Fluorophosphate                                                                         0.2 Al(PO.sub.3).sub.3 -29 AlF.sub.3 -10.2 ZrF.sub.4 -9.8                     BaF.sub.2 -18.3 CaF.sub.2 -12 SrF.sub.2 -8.3 YF.sub.3 -3.5                     MgF.sub.2 -                            0.20                                    3.8 NaF-5 PbF.sub.2 !/ 0.2 Al(PO.sub.3).sub.3 -10 ZrF.sub.4                    -10 BaF.sub.2 -18 CaF.sub.2 -13 SrF.sub.2 -8 YF.sub.3 -                        3.3 MgF.sub.2 -3.5 NaF!                                             8  Bismuth (61 Bi.sub.2 O.sub.3 -26 CdO-13 Ga.sub.2 O.sub.3)/(55 Bi.sub.2                 O.sub.3 -28 CdO-13 Ga.sub.2 O.sub.3 -4 PbO)                                                                           0.19                         9  Lead-Bismuth                                                                           (25 Bi.sub.2 O.sub.3 -57.5 PbO-17.5 Ga.sub.2 O.sub.3)/(28                      Bi.sub.2 O.sub.3 -56 PbO-16 Ga.sub.2 O.sub.3)                                                                         0.21                         10 Aluminate                                                                              (20 Al.sub.2 O.sub.3 -40 Ta.sub.2 O.sub.5 -40 K.sub.2 O)/(30                   Al.sub.2 O.sub.3 -35 Ta.sub.2 O-17.5 K.sub.2 O-17.5 Na.sub.2                   O)                                     0.12                         11 Antimonate                                                                             (50 Sb.sub.2 O.sub.3 -18 Al.sub.2 O.sub.3 -26 K.sub.2 O-3                      Na.sub.2 O-3 Li.sub.2 O/(48 Sb.sub.2 O.sub.3 -18 Al.sub.2                      0.sub.3 -26 K.sub.2 O-6 Na.sub.2 O)    0.18                         12 Fluoride                                                                               (53 ZrF.sub.4 -18 BaF.sub.2 -3 CaF.sub.3 -3 AlF.sub.3 -18                      NaF-5 PbF.sub.2)/(53 ZrF.sub.4 -20 BaF.sub.2 -4 LaF.sub.3 -3                   AlF.sub.3 -20 NaF)                     0.20                         __________________________________________________________________________

EXAMPLE 12

The rotational casting technique was used to prepare Fibers 1, 3 through 7 and 12 of the Table 2 because of their low melt viscosities. In each case, 23 g of cladding glass and 25 g core glass were used to make a preform 6.5 cm long and 1 cm in diameter. The pre-melts for cladding and core glasses were remelted for two hours at a temperature T_(melt). The cladding melt was cast into a mold pre-heated a temperature T_(mold) which was about the glass transition temperature. The mold was then spun and the melt solidified to form a cladding tube. The rotational casting process parameters T_(melt), are given in Table 3 for each of the Fibers from 1, 3 through 7 and 12.

                  TABLE 3                                                          ______________________________________                                         Preform processing and draw parameters used in the                             fabrication of the heavy-metal oxide glass fibers.                             Fiber Preform                                                                  No.   Type         T.sub.melt (°C.)                                                                  T.sub.mold (°C.)                                                                T.sub.draw (°C.)                   ______________________________________                                         1     Germanate    950       330     420                                       3     Germanate    950       380     450                                       4     Germanate    950       310     400                                       5     Tellurate    1050      350     510                                       6     Phosphate    1000      445     625                                       7     Fluorophosphate                                                                             1000      390     535                                       12    Fluoride     800       263     330                                       ______________________________________                                    

The core melt was subsequently poured into the tube to form a preform having a waveguide structure. The preform was then drawn into fiber at a temperature T_(draw) in an electric furnace flushed with a dry argon atmosphere. The fiber was coated with UV-acrylate buffer to preserve its mechanical strength. All fibers obtained had a core diameter of 325 microns and a clad diameter of 525 microns, ±8 microns.

The double crucible technique was applied in the fabrication of Fibers 2 and 8 through 11 because of their high melt viscosities. The double crucible set up consisted of a small platinum crucible concentrically placed inside a larger crucible. The inside diameters of the inner and outer crucibles were 2 cm and 4 cm, respectively. The bottoms of the inner and outer crucibles were tapered to form a nozzle measuring 1.9 mm and 2.2 mm, respectively. The double crucible set-up was placed inside an electric furnace flushed with dry argon gas and was pre-heated at a temperature T_(draw) which was the softening or working temperature of the glass.

The pre-melts for cladding and core glasses were remelted for two hours at a temperature T_(melt). The cladding melt and core melt were cast into the outer crucible and inner crucible, respectively. The melts rapidly cooled down to the working point and were drawn down into fibers having a 325 micron core and a 525 micron clad, ±15 microns. The fiber was coated in-line with a UV acrylate buffer to preserve its mechanical strength.

The double crucible draw parameters are given in Table 4 for each of the Fibers 2 and 8 through 11.

                  TABLE 4                                                          ______________________________________                                         Double crucible draw parameters used in the                                    fabrication of the heavy-metal oxide glass fibers.                             Fiber     Glass                                                                No.       Type         T.sub.melt (°C.)                                                                 T.sub.draw (°C.)                        ______________________________________                                         2         Germanate    1150     825                                            8         Bismuth      1000     475                                            9         Lead-Bismuth 1000     435                                            10        Aluminate    1100     750                                            11        Antimonate   1100     615                                            ______________________________________                                    

EXAMPLE 13

The fibers prepared using the methods of Example 11 were characterized in terms of their optical transmission. Using a Nerst glower, a monochromater, infrared filters and an InSb detector, the fiber loss measurements were carried out at 2.06 microns and 2.94 microns, using the conventional fiber cut-back technique. In the cut-back technique, the light transmission through a long fiber, T_(long), was first measured. The fiber was then cut into a shorter length, and the light transmission through the short fiber, T_(short), was recorded. The fiber attenuation, α, expressed in dB/m was obtained as follows: ##EQU1## when L₂ =length of the long fiber L₁ =length of the short fiber

The measured attenuation for each of the fibers of Example 11 is set forth in Table 5.

                  TABLE 5                                                          ______________________________________                                         Measured attenuation for each of the surgical                                  fibers.                                                                        Fiber   Glass         α(dB/m) at                                                                         α(dB/m) at                               No.     Type          2 microns 2.94 microns                                   ______________________________________                                         1       Germanate     1.1       2.9                                            2       Germanate     0.8       10                                             3       Germanate     0.9       2.7                                            4       Germanate     1.0       2.9                                            5       Tellurate     1.5       2.2                                            6       Phosphate     2.3       7.6                                            7       Fluorophosphate                                                                              0.2       0.5                                            8       Bismuth       3.2       5.1                                            9       Lead-Bismuth  4.6       6.9                                            10      Aluminate     4.8       8.3                                            11      Antimonate    1.9       3.7                                            12      Fluoride      0.1       0.06                                           ______________________________________                                    

EXAMPLE 14

The 325 micron core and 525 micron clad optical fibers prepared using the methods of Example 11 were tested for their power handling capability using an Er:Yag laser and an Ho:Yag laser. The Er:Yag and Ho:Yag lasers emit 250 micro-second pulses of radiation at 2.94 microns and at 2.06 microns, respectively, and deliver a maximum of 500 mJ per pulse in multimode operation. Each fiber under test was carefully cleaned until two good ends were obtained. The laser radiation I_(i) was coupled to the fiber via a CaF₂ lens combination having a 22 mm focal length. The output energy, I_(o), was measured with a radiometer set to measure ten consecutive pulses. A repetition rate of 10 Hz was used. The duration of the laser operation was stretched to a maximum of twenty minutes to avoid damaging the laser cavity. The resulting values of I_(i), I_(o), the fiber length under test, the power coupling efficiency and the condition of each fiber after the test are given in Table 6 for the Er:Yag laser and in Table 7 for the Ho:Yag laser.

                                      TABLE 6                                      __________________________________________________________________________     Power handling characteristics of heavy-metal oxide glass fibers as            compared to                                                                    fluoride glass fiber using an Er:Yag laser.                                                         coupling                                                  Fiber          I.sub.o (mJ)                                                                         efficiency                                                No.                                                                               Glass Type                                                                             I.sub.i (mJ)                                                                       (1 ft length)                                                                        (1 ft length)                                                                        Fiber Conditions                                    __________________________________________________________________________     1  Germanate                                                                              500 340   68%   No damage observed after maximum                                               duration of test of twenty minutes.                 2  Germanate                                                                              500 160   32%   No damage observed after maximum                                               duration of fifteen minutes.                        3  Germanate                                                                              500 360   72%   No damage observed after maximum                                               duration of test of twenty minutes.                 4  Germanate                                                                              500 350   70%   No damage observed after maximum                                               duration of test of twenty minutes.                 5  Tellurate                                                                              500 345   68%   No damage observed after maximum                                               duration of test of twenty minutes.                 6  Phosphate                                                                              500 260   52%   No damage observed after maximum                                               duration of test of twenty minutes.                 7  Fluorophosphate                                                                        500 440   88%   No damage observed after maximum                                               duration of test of twenty minutes.                 8  Bismuth 500 175   35%   No damage observed after maximum                                               duration of test of twenty minutes.                 9  Lead-Bismuth                                                                           500 135   27%   No damage observed after maximum                                               duration of test of twenty minutes.                 10 Aluminate                                                                              500 200   40%   No damage observed after maximum                                               duration of test of twenty minutes.                 11 Antimonate                                                                             500 300   60%   No damage observed after maximum                                               duration of test of twenty minutes.                 12 Fluoride                                                                               500 450   90%   Input ends melted, fiber fractured                                             after 1 min 05 sec.                                 __________________________________________________________________________

                                      TABLE 7                                      __________________________________________________________________________     Power handling characteristics of heavy-metal oxide glass fibers as            compared to                                                                    fluoride glass fiber using an Ho:Yag laser.                                                         coupling                                                  Fiber          I.sub.o (mJ)                                                                         efficiency                                                No.                                                                               Glass Type                                                                             I.sub.i (mJ)                                                                       (1 ft length)                                                                        (1 ft length)                                                                        Fiber Conditions                                    __________________________________________________________________________     1  Germanate                                                                              500 390   78%   No damage observed after maximum                                               duration of test of twenty minutes.                 2  Germanate                                                                              500 412   82%   No damage observed after maximum                                               duration of fifteen minutes                         3  Germanate                                                                              500 410   82%   No damage observed after maximum                                               duration of test of twenty minutes.                 4  Germanate                                                                              500 410   82%   No damage observed after maximum                                               duration of test of twenty minutes.                 5  Tellurate                                                                              500 365   73%   No damage observed after maximum                                               duration of test of twenty minutes.                 6  Phosphate                                                                              500 385   77%   No damage observed after maximum                                               duration of test of twenty minutes.                 7  Fluorophosphate                                                                        500 447     89.5%                                                                              No damage observed after maximum                                               duration of test of twenty minutes.                 8  Bismuth 500 225   45%   No damage observed after maximum                                               duration of test of twenty minutes.                 9  Lead-Bismuth                                                                           500 180   36%   No damage observed after maximum                                               duration of test of twenty minutes.                 10 Aluminate                                                                              500 280   56%   No damage observed after maximum                                               duration of test of twenty minutes.                 11 Antimonate                                                                             500 350   70%   No damage observed after maximum                                               duration of test of twenty minutes.                 12 Fluoride                                                                               500 450   90%   Both input and output ends developed                                           craters after 1 min. 33 sec. power                                             quickly dropped to zero.                            __________________________________________________________________________

The results from these tests indicate that all heavy metal oxide glass fibers of this invention exhibit power handling much longer than the fluoride glass fiber which is absolutely required in laser surgery, although their power transmission efficiency is lower due to their higher attenuation at 2.94 microns and 2.06 microns. The fluoride glass fiber on the other hand could deliver 90% of the laser output power but was considered useless because of its almost instantaneous degradation. The fluoride glass fiber breaks down after only 1-1.5 minutes when coupled to an Er:Yag laser at an output power of about 3 watts, required for many surgical procedures. The power handling ability of the present heavy metal oxide glass fibers is a first time result in this field, insofar as known.

The test results of Table 6 also show that the power transmission of Fibers 2, 8 and 9 was among the lowest because fiber 2 had a high attenuation of 10 dB/m at 2.94 microns. Fibers 8 and 9 on the other hand had lower attenuation but very high refractive indices of 2.43 and 2.46, respectively. These high refractive indices contributed a total of about 35% reflection loss at the two ends of fibers 8 and 9. To increase the coupling efficiencies, the ends of fibers 8 and 9 can be coated with a standard anti-reflection coating that will reduce the total reflection loss of the two fibers to about 10%.

All the heavy-metal oxide glasses of the present invention exhibited transmission in the 90% range between 1 to 3 microns over a 2 mm optical path length and at least 27% over 30.5 cm optical path length. The optical attenuation of the heavy-metal oxide glass fibers increases as the operational wavelength decreases toward 1 micron, since the infrared absorption cut-off edge becomes weaker at shorter wavelengths. This behavior is apparent from the test results of Tables 6 and 7 which show that the fiber coupling efficiency was higher at 2.06 microns than at 2.94 micron. As a result the heavy-metal oxide glass fibers can be efficiently used in conjunction with any surgical lasers covering the 1 to 3 microns operational wavelength region.

It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. 

What is claimed is:
 1. A surgical laser operating in the wave length region of 2.06 microns up to 4.5 microns for power transmission and including an optical fiber for transmitting laser light, the improvement whereinsaid optical fiber has a solid core formed of a glass comprising a mixture of (1) GeO₂ as class former and (2) at least one dooant selected from the group consisting of oxides and halides of heavy metal cations having an atomic weight above 73, said mixture constituting at least 73 mol % of said glass.
 2. A surgical laser operating in the wavelength region of 2.06 microns up to 4.5 microns for power transmission and including an optical fiber for transmitting laser light,said optical fiber containing GeO₂, no more than 27 mol % of lightweight oxides having a lighter weight than then GeO₂, and less than 1 ppm of H₂ O and having a glass transition temperature Tg of at least 290° C., and having a coupling efficiency of at least 20% over an optical path length of 30.5 cm, said glass having a critical cooling rate Rc of less than 5.5° C./min, said optical fiber constituting means for delivering at least 1.35 watts of laser power continuously for at least five minutes and without failure.
 3. A surgical laser according to claim 2, wherein said optical fiber has a length of at least 15.25 cm.
 4. A surgical laser according to claim 2, wherein said optical fiber has a length of at least 30.5 cm.
 5. A surgical laser according to claim 2, said glass comprising 18-80 mol % GeO₂ and at least one dopant selected from the group consisting of PbF₂, NaF, AlF₃, SrF₂, CaF₂ and PbCl₂, and optionally at least one additional dopant selected from the group consisting of Pbo, TeO₂, ZrO₂, La₂ O₃, BaO, Bi₂ O₃, Sb₂ O₃, As₂ O₃ and ZnO₂.
 6. A surgical laser according to claim 5, wherein said glass contains no more than 0.1 ppm of H₂ O.
 7. A surgical laser according to claim 6, further comprising a dopant selected from the group consisting of ZnO and TiO₂.
 8. A surgical laser according to claim 5, further comprising a dopant selected from the group consisting of ZnO and TiO₂.
 9. A glass fiber according to claim 2, wherein said glass contains no more than 0.1 ppm of H₂ O.
 10. A substantially water-free glass fiber for use with a laser operating in the mid-infrared wavelength region of up to 3.5 microns, said fiber having a solid core and a cladding, said core having a glass transition temperature Tg of at least 290° C., and comprising a heavy-metal oxide and no more than 27 mol % of compounds of lighter weight than GeO₂, and being capable of transmitting IR at about 1.0 to 3.0 microns at least 50% over an optical path length of 30.5 cm, said fiber having a length of at least 15.25 cm and said glass having a critical cooling rate Rc of less than 5.5° C./min.
 11. A glass fiber according to claim 10, wherein said optical fiber is capable of transmitting IR at 1.0 to 3.0 microns of at least 40% over a 30.5 cm optical length.
 12. A glass fiber according to claim 10, wherein said glass has a Tg of at least 315° C. and said optical fiber has a coupling efficiency of at least 35% over a 30.5 cm length.
 13. A glass fiber according to claim 12, wherein said Rc is less than 3.0° C./min and said coupling efficiency is at least 50% over a 30.5 cm length.
 14. A glass fiber according to claim 10, wherein said glass further comprises an amount sufficient of a fluoride dopant to minimize the water content of said glass.
 15. A fiber according to claim 10, having a length of at least 30.5 cm, and a coupling efficiency of at least 50% over a 30.5 cm length.
 16. A method of forming a substantially water-free glass fiber according to claim 10, comprisingpassing a reactive halogen gas over chemical components from which said glass is formed, including at least one fluoride or chloride dopant, at a temperature of less than 100° C. to effect drying thereof, and weighing, batching and melting said chemical components in a dry atmosphere containing less than 0.1 ppm of water.
 17. In a method for power transmission comprising passing laser light in a wavelength region of 2.06 up to 4.5 microns through an optical fiber, the improvement wherein said optical fiber is a glass fiber according to claim
 10. 18. A glass fiber according to claim 10, said glass comprising 18-80 mol % GeO₂ and at least one dopant selected from the group consisting of PbO, PbF₂, NaF, AlF₃, TeO₂, ZrO₂, La₂ O₃, BaO, Bi₂ O₃, Sb₂ O₃, As₂ O₃, SrF₂, ZnO₂, CaF₂ and PbCl₂.
 19. A glass fiber according to claim 10, said glass comprising 10-85 mol % GeO₂ and at least one dopant selected from the group consisting of PbO, PbF₂, NaF, AlF₃, TeO₂, ZrO₂, La₂ O₃, BaO, Bi₂ O₃, Sb₂ O₃, As₂ O₃, SrF₂, ZnO₂, CaF₂ and PbCl₂, and further comprising a dopant selected from the group consisting of ZnO and TiO₂.
 20. A substantially water-free glass fiber according to claim 10, wherein said core comprises GeO₂ as glass former and at least one heavy metal dopant selected from the group consisting of oxides and halides of cations having an atomic weight above 73, and wherein said GeO₂ and said at least one heavy metal dopant constitute at least 73 mol % of said glass. 